There is evidence that carotid artery imaging techniques can be used to detect disease and classify subjects that will benefit from interventions. Because Magnetic Resonance Imaging (MRI) can acquire images with a variety of different tissue contrasts, it presents an advance over x-ray techniques in the ability to discriminate between different types of soft tissue and the potential to discriminate between important atherosclerotic plaque components. Turbo spin echo (TSE) MRI pulse sequences, with efficient acquisition of T1, T2 and proton density weighted images have become very useful in the evaluation of cervical carotid artery disease. Current high-resolution three-dimensional (3D) TSE techniques could also be valuable in carotid MRI. However, motion artifacts that arise due to the relatively long data-acquisition time of current TSE pulse sequence methods result in an unacceptably high fraction of non-interpretable images. As a result, TSE techniques result in obvious blurring or ghosting artifacts that greatly reduce the quality of the images.
Methods can be implemented to reduce artifacts caused by poor acquisition time. For example, applying parallel imaging techniques may be used to reduce acquisition time and thereby reduce the likelihood of motion artifacts. Parallel imaging techniques can also be used to compensate for artifacts due to swallowing and other neck muscle movements. Reduced field of view sequences with more efficient double inversion blood suppression have also shown reduced susceptibility to these types of motion artifacts. Single shot sequences such as Half Fourier Acquisition Single shot Turbo spin Echo (HASTE) can also be used to reduce acquisition time and hence the susceptibility to certain motion artifacts. Navigators and dedicated motion detection RF coils have been shown to be successful in detecting and rejecting data corrupted by swallowing. Additionally, using average specific phase encoding ordering or comparing adjacent sets of k-space data lines are alternative methods for detecting data corrupted by patient motion.
Other MRI scanning methods, such as Gradient Echo (GRE) methods, are used for faster imaging to reduce more complex motion artifacts, such as ones that are caused by the movements of the cardiac cycle. These may include synchronizing data acquisition with the heart cycle to insure controlled capture across all data points. GRE-based pulse sequences can have a very short Repetition Time (TR) (<30 ms) and can therefore be efficiently designed as cine sequences to obtain complete image measurements at multiple cardiac phases in the same acquisition. Pulse sequences such as cine-GRE or cine-Phase-Contrast (PC) have been used to show carotid artery motion and cyclic blood flow. However, gradient echo techniques are limited in their ability to create acceptable images of soft tissue due to the above noted short TR time and typically worse signal-to-noise ratio performance.
TSE techniques, on the other hand, generate detailed images of soft tissue. However, the same methods to compensate for motion artifacts that are caused by the movements of the cardiac cycle are not used with TSE techniques. This is in part because of the relatively long TR required by TSE techniques (>500 ms). Acquisitions synchronized with the heart cycle which use TSE techniques typically only obtain one image at a single cardiac phase. Accordingly, synchronized acquisitions using TSE techniques are generally only used to reduce the effect of cardiac motion by acquiring data at a point during a specific time period of the cardiac cycle, e.g., a point during the diastole period. Further, synchronization lengthens the acquisition time due to the need to acquire multiple scans at each specific point in the cardiac cycle. This additional time results in increased susceptibility to swallowing and respiratory motion artifacts. Because of this, synchronized acquisitions are rarely used in a TSE environment. This problem can be improved somewhat by limiting gating to just the central portion of k-space, however, the effect of a variable TR on the resulting image is still unacceptable.